As countermeasures against the flowing out of contaminants in the storm overflow chamber, a vertical control plate 6 as described in Patent Document 1 (JP 2004-238833 A) (refer to Abstract and FIG. 1) is known. The vertical control plate 6 generates a vortex near an opening of an intercepting pipe 3. Floating contaminants 5 are drawn into the vortex, and then contaminants 5 are drawn into the intercepting pipe 3.